When Will the Next Ice Age Begin?

By ANDREW C. REVKIN

Published: November 11, 2003

The maxim ''what goes around comes around'' applies to few things more aptly than ice ages. In a rhythm attuned to regular wiggles in Earth's orbit and spin, 10 eras of spreading ice sheets and falling seas have come and gone over the last million years.

Through that span, in fact, the cold spells have so dominated that geophysicists regard warm periods like the present one, called the Holocene, as the oddities. Indeed, the scientific name for these periods -- interglacials -- reflects the exceptional nature of such times.

The next ice age almost certainly will reach its peak in about 80,000 years, but debate persists about how soon it will begin, with the latest theory being that the human influence on the atmosphere may substantially delay the transition.

This is no mere intellectual exercise. The equable conditions of the Holocene, which has lasted 10,000 years so far, have enabled the flowering of agriculture, technology, mobility and resulting explosive population growth that has made the human species a global force.

Any substantial climate shift is likely to pose enormous, though probably surmountable, challenges.

Just 30 years ago, after a prolonged global cool spell, many climate scientists, including some now focused on global warming, posited that Earth might already be seeing the onset of the next big chill.

Evidence from sea sediments and other sources had consistently put the duration of the previous warm spell at about 10,000 years, and it was presumed that this provided at least a rough hint of the longevity of the current interglacial.

The notion that cooling was imminent was challenged several years ago. Some scientists gleaned more details about the previous warm spell, which occurred 130,000 years ago, and concluded that it lasted twice as long as they had previously estimated -- 20,000 years instead of 10,000.

Others have proposed that an earlier warm era that lasted even longer -- 30,000 years -- was a better model for the Holocene. But many experts still say they are convinced that the current warmth should, under the influence of orbital cycles alone, near an end ''any millennium now,'' as Dr. Richard A. Muller, a physicist at the University of California at Berkeley, puts it.

But the planet is feeling a new influence, that of people. Humans may delay the dawn of the next ice age by a millennium or two, or even longer, many climate experts say, as Earth's long-buried stores of coal, oil and other carbon-rich fossil fuels are burned, releasing billions of tons of carbon dioxide and other heat-trapping greenhouse gases.

That insulating blanket has a bigger climatic influence than the slight flux in incoming solar energy from changes in Earth's orientation relative to the Sun, said Dr. James A. Hansen, the director of NASA's Goddard Institute for Space Studies.

''We have taken over control of the mechanisms that determine the climate change,'' he said.

Other scientists, while agreeing with this thesis for the short term, say that eventually the buffering properties of the atmosphere, ocean and Earth will restore balance, returning most of the liberated carbon to long-term storage and allowing the orbital rhythm once again to dominate.

''Orbital changes are in a slow dance leading to a peak 80,000 years from now,'' said Dr. Eric J. Barron, the dean of the College of Earth and Mineral Sciences at Penn State. ''I can hardly imagine that human influences won't have run their course by that time.''

It may seem that human-driven global warming, although perhaps a disaster on the scale of centuries, may be a good thing in the long run if it fends off the next ice age awhile.

But many climatologists note that the complex interplay of greenhouse gases, orbital shifts and other influences on climate remain poorly understood. In fact, some experts say, there is a chance that human-induced warming could shut down heat-toting ocean currents that keep northern latitudes warmer than they otherwise would be. The result could be a faster descent into glacial times instead of a delay.

Chart: ''Global Cooling'' Past glacial advances and retreats are concentrated in just a few periods of time. Technically, we are now in an ''ice age,'' if it is defined by long, generally cool periods during which glaciers grow and melt. The modern climate is a relatively brief warm period. Charts shows the glacial advances and retreats over 600 million to 800 million years ago to present. During the last 2 million years, more than 20 glacial advances and retreats have occurred. (Source by Illinois State Museum)